Neck down t shirt, change yarn size

Knitting Pure & Simple neck down v neck t shirt for women. This pattern uses light worsted or dk yarn. Size 6 needle. 20 st = 4 inches. I have knitted 1/3 of the pattern and love it and it fits. I used concerto linen yarn. I want to make a second one using Juniper Moon Findley yarn. The size of the yarn is much smaller. I probably can combine multiple skeins to get to the same size yarn but I am thinking a single strand would be so light and comfy. I can see knitting a test gauge block and getting a larger number of stitches for 4 inches I checked the findley skein and it says size 2 needle 30 stitches for 4 inches. I am concerned about the part of the pattern where working the sleeves for example knit 5 rounds then decrease row. (at marker k1 ssk, knit round to last 2 stitches k2 tog) it seems I don’t know now to change those instructions to make it work. I suppose also right from the beginning with the increases that create the sleeves. It seems I could knit a lot of rounds and have a garmet that won’t fit. Any formula? or suggestions?

Welcome to KnittingHelp!
That’s quite a change in gauge. You might consider looking for a similar pattern that works with the thinner yarn.
You certainly should knit a gauge swatch to see exactly how many sts you get to 4 inches with the Findley on whichever size needles. You can see if you like the drape and looseness of the yarn.

The conversion for the cast on and straight knitting is straightforward (your gauge x sts cast on) divided by the given gauge. So, for example for a 100 stitch cast on in the pattern (30 x 100)divided by 20 = cast on 150sts. That’s one third more sts than the pattern calls for and you can use that number throughout.
For areas of shaping you’d have to consider the width of the sleeve that you want at that point in the shaping and convert from inches in the pattern to sts at your gauge. That goes for the increases at the V-neck too.
You’ll have to determine if the placement of sleeve and neck shaping will work with the new yarn, too.

thank you for your reply. I understand your directions. I will try finding a pattern for the thinner yarn and also will knit up a swatch and test. I have a findley pattern booklet, perhaps there is a pattern I can convert to the v neck style. I like top down as I can try it on as I go. Thanks again.